1. Field of the Invention
The present invention relates to a multi-input multi-output (MIMO) antenna with a multi-band characteristic, and more particularly, to a multi-input multi-output antenna with a multi-band characteristic which includes a plurality of MIMO antenna, each having a pair of antenna elements, to support multiple bands, and is capable of guaranteeing high antenna efficiency for different bands by minimizing an interference between antenna elements of each MIMO antenna to improve an isolation characteristic.
2. Description of the Related Art
The current mobile communication technology is transitioning from the third generation (3G) into the fourth generation (4G) such as LTE (Long Term Evolution) or Wibro (or Mobile WiMAX) and a variety of communication systems for supporting a band used in 4G and guaranteeing mobility are coming on.
One of these communication systems is a multiple-input multiple-output (MIMO) system using a plurality of antennas. MIMO is a new and attractive approach to overcome problems of wireless communications, such as signal attenuation, interference increase, spectrum limitation and so on.
MIMO uses a plurality of antennas to provide diversity of the antennas, thereby consuming no additional wireless frequency while doubling a data processing speed and increasing a band range and reliability.
MIMO is an innovative multi-dimensional approach to transmit/receive two or more individual data streams through one wireless channel and allow a communications system to provide two times or more a data transfer rate per channel. By allowing the simultaneous transmission of data streams, MIMO can increase wireless data capacity several times without using any additional frequency spectrum.
The highest processing speed of the MIMO system can be increased by a multiple corresponding to the number of signal streams transmitted by a wireless channel. An MIMO signal is also called a “multi-dimensional signal” since a plurality of signals can be transmitted from different wireless apparatuses and antennas.
The MIMO system may also have the following demerits. First, the MIMO system requires a plurality of antenna elements. Accordingly, for example, a mobile terminal requires a plurality of antennas contained in a space relatively smaller than a base station. The use of the plurality of antennas may lead to a coupling effect between the antennas, which may result in signal distortion or cancellation and hence deterioration of signal reception sensitivity. Therefore, it is very important to remove such a coupling effect.
In order to reduce the coupling effect and prevent signals emitted from each antenna from having no effect on other antennas, an isolation has to be increased by separating the antennas by a predetermined distance so that a radiation pattern range occupied by one antenna element does not overlap with radiation pattern ranges occupied by other antenna elements. However, under current situations where mobile terminals are growing and growing smaller and accordingly MIMO elements are growing and growing smaller, it is nearly impossible to guarantee an isolation between antenna elements, which may result in overlap of radiation patterns of the antenna elements and hence signal attenuation and deterioration of signal reception sensitivity due to interference between signals emitted from the antenna elements.
In recent years, in order to overcome this problem, an approach of adding a circuit-short line for circuit-shorting between antenna elements having different power feeding points has been employed, as shown in FIG. 1. In this approach, since a signal induced from a firs antenna element into a second antenna element is in the reverse direction to a signal of the first antenna element, the signal of the first antenna element and the signal of the second antenna element are cancelled each other in the circuit-short line to thereby increase the isolation so that this approach can be applied to small-sized mobile terminals.
However, the above-mentioned approach has a demerit of change in an antenna element band characteristic to significantly reduce a band width.
In addition, 4G communication technologies have recently required MIMO antennas to support a plurality of bands. However, as described above, the current MIMO antenna implementation techniques are hard to meet requirements of high isolation and sufficient band width. Further, as the number of antenna elements required for implementation of the plurality of bands increases, the above space problem of mobile terminals may be more serious.
Accordingly, there is a need of an improved MIMO antenna implementation technique using a plurality of antenna elements to overcome the above-mentioned problems.